1. Field of the Invention
This invention relates to apparatus for generating a signal which has spaced first and second frequency channels of which each has an independently predetermined power, the power of the first channel being substantially below that of the second, and each channel carrying frequency modulation with an independently predetermined modulation rate and deviation. Such signals are particularly useful as test signals for testing adjacent channel rejection of radio receivers.
2. Description of Related Art
Adjacent channel rejection is a measure of the capability of a radio receiver to ignore strong signals in an adjacent channel (the second channel) while receiving a weak signal in the wanted channel (the first channel). This situation might arise where a radio is receiving transmissions at the very limit of one transmitter's range, while a nearby transmitter is transmitting on the next frequency channel. Typical channel spacings are 12.5 kHz, 20 kHz and 25 kHz. Typical radio receivers using frequency modulation allow peak frequency deviations of 2.5 kHz for 12.5 kHz spacings, and 5 kHz for 20 kHz or 25 kHz spacings.
Conventional equipment for testing the adjacent channel rejection of such radio receivers is illustrated in FIG. 1 of the accompanying drawings. The wanted signal channel comprises a frequency modulation source 14 connected to modulate a radio frequency source 12, whose output is fed by way of a radio frequency attenuator 15 to a combiner 16. The adjacent signal channel, in which the power of modulation is substantially higher, consists of a similar frequency modulation source 13, connected to modulate a relatively high purity radio frequency source 11 whose output is fed directly to the combiner 16. The composite test signal 17 is taken as the output from the combiner 16, and is illustrated diagrammatically in FIG. 2 of the accompanying drawings.
FIG. 2 illustrates the power spectrum in the region of the wanted channel at frequency F.sub.W, and the adjacent channel at frequency F.sub.ADJ. As a typical specification for the adjacent channel rejection may be 70 dB, the generator used for the high power signal, RF source 11, must have very good spectral purity. It must be at least 20 dB better in sideband performance at an offset corresponding to the channel spacing than the rejection specification to be measured. The signal generator, RF source 12, for the wanted channel need not have such a high specification for spectral purity.
The test is typically performed by adjusting the level of the wanted channel generator, modulated with typically 60% maximum deviation at 1 kHz, to give a 12 dB SINAD (signal to noise and distortion radio) on the receiver under test. This radio frequency level is now increased by 6 dB. The adjacent channel power, modulated at typically 60% maximum deviation at 400 Hz, is increased until the receiver returns to 12 dB SINAD. The radio of these two powers, represented in FIG. 2 as the adjacent channel power ratio 20, is known as the adjacent channel rejection.
The problem with conventional test signal generators is that they require the use of two signal generators 11, 12. This yields an uncertainty in the power difference which equals the sum of the uncertainties in the power level of each generator, the gain of the attenuator 15, and the balance of the combiner 16. The resulting uncertainty can be very large.